Two new studies out today show both the incredible promise of stem cell research and its current limitations.

In one, published in the journal Nature, researchers showed that they could repair damaged hearts by injecting these versatile stem cells into macaque monkeys. Heart disease is the leading cause of death, and if the same process can work in people, it could benefit hundreds of thousands a year.

In the other study, published in Science Translational Medicine, five men were able to regrow leg muscles destroyed by accidents or military service. The researchers, from the University of Pittsburgh, inserted into the men's muscles a "scaffold" of muscle tissue from a pig. Through aggressive physical therapy right after the surgery, the men's own stem cells were encouraged to populate the scaffold and substantially rebuild their leg muscles.

Nothing had been able to help these men before, including multiple surgeries and years of physical therapy, said Stephen Badylak, the study's senior author.

"Frankly, most of these patients have been through hell," he said at a Tuesday news conference.

David Scadden, a physician and co-director of the Harvard Stem Cell Institute, said he was impressed with the rigor and promise of both studies.

It's long been a goal of stem cell research to figure out how to help the body regrow damaged tissue, he said, and both studies mark a significant step toward that goal.

Both studies also showed that stem cells respond to cells around them, he said, with the heart cells learning to beat in sync with the monkey cells and the muscle cells learning to go where they were needed. "Once the cells get to a certain point, it appears they can then follow the lead of their neighbors," he said.

Stem cells gained public notoriety about a decade ago when then-President George W. Bush banned the creation of new stem cell lines from human embryos.

Early embryos are the best source of stem cells, because an embryonic cell can transform into any type of cell – in the liver, blood, brain, etc. More recently, researchers have learned how to turn adult cells, such as from the skin, back into this super-versatile state, but such induced pluripotent stem cells still have limitations.

In the heart study, University of Washington researchers led by Charles E. Murry took human embryonic stem cells, turned them into muscle cells and then inserted them into the damaged hearts of seven pigtail macaque monkeys. The cells became virtually indistinguishable from the monkey's own cells, beating along with them.

"I really believe we can grow back people's hearts," said Murry, a pathologist and co-director of the Institute for Stem Cell and Regenerative Medicine at the University of Washington. He hopes to be able to start trials in people in about four years, after meeting all of the federal requirements.

In the second study, researchers did not need to implant any cells, but relied on the patients' own stem cells.

Both studies also showed how difficult it is to manipulate stem cells, and how long it will be until stem cell therapies become a routine part of medical care, said Timothy Caulfield, a science policy expert at the University of Alberta in Canada.

Right now, the only type of stem cell therapy that is commonplace and medically proven is bone marrow transplantation to treat certain types of blood cancers, Caulfield said.

But unscrupulous clinics – primarily in other countries, but some in the USA – have been offering what they call stem cell therapies for a wide range of diseases.

As legitimate research gets closer to providing widespread benefits, it will get even harder to tell the frauds from the legitimate providers, Caulfield said.

• The approach should be specific to a particular condition, not effective against multiple problems.

• The provider should discuss risks of the procedure along with potential benefits.

• The provider should make it clear that the procedure is experimental, not routine (other than bone marrow transplants for cancer).

• The costs of the therapy should be paid by a research institution if it is part of a research trial or insurance for a bone marrow transplant, so be wary of someone who charges significant sums for treatments.